Recent years have seen the development of biometric authentication technology for authenticating a user of an apparatus or system based on a biometric image representing the user's biometric information such as a palm or finger vein pattern or a fingerprint or palm print. A biometric authentication device using such biometric authentication technology acquires, as a biometric input image, a biometric image representing, for example, the biometric information of the user who wishes to use the biometric authentication device. Then, the biometric authentication device compares the user's biometric input information represented by the biometric input image with registered biometric information, i.e., biometric information prestored in a biometric image for each registered user. If the result of the comparison indicates that the biometric input information matches the registered biometric information, the biometric authentication device authenticates the user as being a legitimate registered user. The biometric authentication device then permits the authenticated user to use the apparatus in which the biometric authentication device is incorporated or some other apparatus connected to the biometric authentication device.
A general problem with the biometric authentication is that the biometric information represented by the registered biometric image does not always match the biometric information represented by the biometric input image even when the biometric information is one acquired of the same person. Possible causes are that the person's body part carrying the biometric information is differently positioned on the sensor at the time of matching than at the time of registration, or the condition of the body part carrying the biometric information is different. If the condition of the body part is different at the time of matching than at the time of registration, the feature quantity extracted from the biometric image acquired at the time of matching, for example, may differ from the feature quantity extracted from the biometric image acquired at the time of registration.
For example, when using a fingerprint as the biometric information, the degree of skin roughness or dryness of the finger surface or the degree of wetness of the finger surface due to moisture such as sweat, water, or hand cream may be different at the time of matching than at the time of registration. In certain instances, a portion of the fingerprint image acquired at the time of matching may be blurred, and the feature quantity extracted from such a fingerprint image may differ from the feature quantity extracted from the fingerprint acquired at the time of registration.
In this case, since the feature quantities extracted from the respective images do not perfect match, the accuracy of matching may degrade.
In view of the above situation, techniques have been developed that judge whether the biometric information represented by the acquired biometric image satisfies a given quality level (for example, refer to Japanese Laid-open Patent Publication No. 2002-298126, Japanese Laid-open Patent Publication No. 2002-208001, Japanese Laid-open Patent Publication No. 2004-86463, and International Publication Pamphlet No. WO2004-102481).
One such prior art discloses an apparatus that determines whether or not the acquired fingerprint image is suitable for use for authentication, by first checking whether a fingerprint formed from ridges and valleys exists in each of the regions into which the fingerprint input image has been segmented and then judging the condition of the skin surface in each segment region. The degree of connectivity of light/dark pixels and the ratio of light/dark pixels are used to determine the presence or absence of a fingerprint and judge the condition of the skin surface, respectively.
Another prior art discloses a fingerprint matching apparatus which calculates the degree of lightness from image data acquired by a fingerprint sensor and, based on the calculated degree of lightness, judges the quality of the image represented by the image data.
Still another prior art discloses a fingerprint matching apparatus which, from fingerprint image data acquired of a subject, generates edge extraction image data that enhances the ridges of the acquired fingerprint or gradation direction extraction image data that indicates the direction of gradation change on a pixel-by-pixel basis. The fingerprint matching apparatus then determines whether or not to register the image data, based on the amount of displacement between the centroid of the edge image and the center of the image or on the total number for each direction pattern that is computed from the gradation change direction pattern for each sub-region of the gradation direction extraction image data.
Yet another prior art discloses a biometric information detection apparatus that judges the condition of an acquired image based on the average value and standard deviation of the pixel values taken over a biometric information region defined on the acquired image.